MINOR: buffer: replace bi_fast_delete() with b_del()

There's no distinction between in and out data now. The latter covers
the needs of the former and supports wrapping. The extra cost is
negligible given the locations where it's used.
diff --git a/src/proto_http.c b/src/proto_http.c
index 37360bf..85c2391 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -3385,7 +3385,7 @@
 		FLT_STRM_CB(s, flt_http_reply(s, txn->status, chunk));
 		co_inject(res->chn, chunk->str, chunk->len);
 		/* "eat" the request */
-		bi_fast_delete(req->chn->buf, req->sov);
+		b_del(req->chn->buf, req->sov);
 		req->next -= req->sov;
 		req->sov = 0;
 		s->req.analysers = AN_REQ_HTTP_XFER_BODY | (s->req.analysers & AN_REQ_FLT_END);